It is generally known that cement products have a number of technical and economical advantages as compared with other constructional materials. However, in many cases the use of cement products is limited due to their insufficient durability and insufficient longevity that are caused by formation of unstructured shrinkage cracks.
Formation of unstructured shrinkage cracks is caused by that small cracks (so called hair cracks) are formed during the production of cement products from cement mortar. Further, when one-time or constant load is applied to an article made of a cement product or by the action of the weight of the article itself the hair cracks begin to propagate, their amount and their size enlarge and they transform into larger shrinkage cracks causing detriment in durability and destruction of the article made from cement product. By preventing the propagation of hair cracks one can achieve an increase in effective durability of a concrete product and promote an increase of its longevity.
In order to additionally increase physical and mechanical durability of cement products different reinforcing components, such as metal reinforcement, mineral elements and synthetic fibers are used. Therewith the synthetic fibers are known to be most effective to prevent the development of unstructured shrinkage cracks from hair cracks as they are the thinnest ones of the above listed types of reinforcement elements and their size is comparable to that of hair cracks.
Thus the three-dimensional reinforcement of a cement product with synthetic fibers is able to compensate the main drawbacks of concrete products such as shrinkage cracking formation, poor tensile strength and high brittleness at destruction. Therefore a cement product comprising a fiber component shows higher collapsing strength, tensile strength, shear strength, impact resistance, fatigue strength, crack strength, cold endurance and watertightness as compared to a cement product which does not comprise reinforcing synthetic fiber.
However, not all synthetic fibers are equally suitable to be used in cement products. First of all, such parameters as durability, deformability, chemical resistance of the reinforcing material and its adhesion to water-cement suspension shall be taken into account. Preferably, the properties of a synthetic fiber used for three-dimensional reinforcement are such that the use of these fibers could allow avoiding, partially or completely, the use of metallic reinforcing elements in the cement product since the use of metallic reinforcement leads to the increase of the weight of a cement product as well as the production costs. Particularly, in order to ensure effective prevention of hair crack formation by use of a synthetic fiber as a component of a cement product, important is that these fibers would be uniformly dispersed within the whole volume of cement mortar since only in such case hair crack formation can be prevented within the whole volume of a cement product.
Patent RU 2274618 discloses a cement product (concrete) reinforced with metallic and synthetic fibers and comprising pozzolana components and a filler having particle size not higher than 10 mm. Synthetic fibers constitute 0.1 to 3 vol. % of the concrete after the setting. Preferably, the synthetic fibers have thickness of less than 80 μm, length between 1.5 mm and 12 mm and preferably consist from polyethylene and polypropylene.
Patent RU 2245858 discloses a cement product (concrete) comprising organic fibers which may consist from polyethylene including high density polyethylene, polypropylene and other synthetic polymers. In order to strengthen the adhesion of organic fibers a substance selected from silica, precipitated sodium carbonate, phosphate, latex polyvinyl alcohol and antifoaming agent is additionally introduced into the cement mortar intended for the production of said concrete. Said concrete additionally further comprises reinforcing fillers which represent metallic fibers or fibers made from other inorganic materials.
The use of polyolefin synthetic fibers as disclosed in the above mentioned patents does not allow avoiding completely the use of metallic reinforcement and inorganic fibers in a cement product. Because of that the production of cement products as disclosed in RU 2274618 and RU 2245858 is expensive and the mass of the product obtained increases due to the inclusion of metallic reinforcing component. One of the reasons leading to the necessity of use of metallic reinforcement in the products disclosed in said patents is the fact that the fibers are not dispersed uniformly within the volume of a cement product and may not thus work effectively. It is due to the fact that surface tension of the fibers used according to said patents is lower than surface tension of water and synthetic polyethylene fibers cannot therefore be homogenously dispersed in aqueous medium of cement mortar.
Better results can be achieved if fibers are used which have been surface-treated so that to increase the surface tension of such fiber and to make its surface uniformly hydrophilic. Owing to such treatment the fibers disperse uniformly within the volume of cement mortar.
So, patent RU 2074153, which is the nearest prior art for the present invention, discloses bunches of synthetic fibers for disperse reinforcement of cement based articles. According to said patent, bunches of synthetic fibers comprising 10 to 106000 individual fibers per a bundle are introduced into cement mortar. Before being introduced into cement mortar, the bunches of synthetic fibers are treated with wetting agent in order to increase surface tension of individual fibers. In order to further increase the surface tension, along with treating with wetting agent corona discharge treatment of fibers may also be applied. Individual synthetic fibers according to said patent represent polyolefin fibers, preferably polyethylene or polypropylene fibers.
However, although the synthetic fibers used according to patent RU 2074153 allow avoiding the use of metallic reinforcement, cement products with such fibers do not show high durability. This is due to the fact that surface treatment of fibers with hydrophylizing agents cannot provide for uniform surface tension all over the whole surface of a fiber. Moreover, when the whole bunch of fibers is treated as it is disclosed in RU 2074153 some individual fibers can remain untreated which makes dispersing of such fibers in a cement product even worse.
Therefore, there still remains a necessity to improve further the hydrophilic characteristics of fibers, such as the strength of surface tension and uniformity of strength of surface tension all over the surface of a fiber.
Thus, the object of the present invention is to provide a synthetic fiber for three-dimensional reinforcement of a cement product, which fiber shall have uniformly hydrophilic surface with high strength of surface tension that would allow such fiber to disperse uniformly within the volume of a cement product. The fiber shall therewith possess good durability characteristics such as high tensile strength, high modulus of elongation and low elongation at break.
It was found surprisingly that the object of the present invention may be achieved by providing synthetic fiber which represents coaxial coat-core fiber wherein the core comprises thermoplastic polymer, preferably polyethylene, polypropylene, poly-4-methyl-1-pentene or polyethylene terephthalate and the coat comprises thermoplastic polymer resistant to pH>11 and also comprises hydrophilic and/or surface active substances in an amount of 3 to 30% of the total weight of the coat, or the coat comprises thermoplastic polymer resistant to pH>11 grafted with active or unsaturated monomers in an amount of 3 to 25% of the total weight of the coat, wherein in both cases the surface tension of the coat of not less than 45 mN/m is achieved.